MARIE Assembly If-Then

Question

Pseudocode:

if x > 1 then
   y = x + x;
   x = 0;
endif;
   y = y + 1;

I am tracing the MARIE code below based from the pseudocode:

ORG 100
IF,     LOAD X
        SUBT ONE / What is this for?
        SKIPCOND 800
        JUMP ENDIF
THEN,   LOAD X
        ADD X
        STORE Y
        LOAD ZERO
        STORE X
ENDIF,  LOAD Y
        ADD ONE
        STORE Y
        HALT
X,      DEC ?
Y,      DEC ?
ONE,    DEC 1
ZERO,   DEC 0

Why is the SUBT ONE needed there?

Solution 1

It does the comparison by subtracting 1 from x, leaving the result in the accumulator. We can then use a conditional branch on whether the resulting value in AC is zero, positive or negative.

Look up what SKIPCOND 800 does: How does `Skipcond` work in the MARIE assembly language?

Unlike most architectures where add/subtract instructions set flags and conditional branches test them, MARIE's conditional branch instruction is a test-and-branch, like MIPS bgtz / beq with $zero / bltz

Solution 2

I think the reason they add

SUBT ONE 

is because we don't have a skip condition for x > 1, but we do have a skip condition for x > 0, which is

skipcond 800 / meaning X > 0

Since that's the case, I think they just subtracted one from both sides, making them both equal. (x - 1) > (1 - 1) / same as (x - 1) > 0. from here, we can use skipcond.

That's my best educated guess for why that's included. Hope this helps five years later.

Comments

[Peter Cordes]

Yup, this is correct, and a more accurate description than @starblue's answer. Skipcond tests the value in AC, and doesn't care about the result of a previous subtraction (How does `Skipcond` work in the MARIE assembly language?)

[Peter Cordes]

Unlike many other assembly languages / ISAs, MARIE doesn't have flags. It's more like MIPS, where SKIPCOND is a test-and-branch on AC. Since we're using it on the SUBT result, the logic of testing the SUBT result is correct, but the mechanism is explained wrong. (How does Skipcond work in the MARIE assembly language?). I decided to go ahead and edit the answer to fix it.